The present invention relates to an automatic focusing apparatus and pattern imaging system, and more particularly to an automatic focusing apparatus and pattern imaging system for inspecting the patterns formed on semiconductor subtrates.
With progress of the photolithograph, various types of patterns, such as a pattern of a metal layer formed on the semiconductor substrate, must be formed with great precision. The same thing is true for pattern inspection, that is, it must be determined whether or not the pattern has defects, the pattern size is proper, the distance between adjacent patterns is proper, the pattern is distorted, the pattern quality is satisfactory, or the pattern is displaced from its correct position etc.
An inspecting system for effecting such inspection thus far employed takes a photograph of a pattern compares the photograph with a given reference of the pattern. This inspecting system, however, has inevitably a problem that a flatness of the surface of the substrate with a pattern formed thereon greatly influences precision of the pattern image. More specifically, if the surface of the substrate is wavy or rugged, a distance of the pattern from an optical system in an imaging apparatus varies. The result is that an image point of the pattern varies in the imaging system.
Therefore, the pattern image is blurred. To avoid this, the conventional system is provided with an automatic focusing apparatus which detects an out-of-focus in the optical system and corrects the same on the basis of the detected one. The automatic focusing apparatus uses an airomicro type sensor for sensing an out-of-focus of the pattern. However, the airomicro type sensor has a low detecting accuracy and a slow sensing response. The sensor requires an air compressor for feeding compressed air. The use of the sensor, therefore, makes the pattern inspecting system bulky.
Further, conventionally, the pattern imaging system and the focusing system each need an optical systems. Therefore, the whole system is bulky, complicated and costly.